1. Field of the Invention
The invention relates to an X-ray image intensifier tube, comprising an entrance section which includes an entrance window and an entrance screen with an entrance luminescent layer, and a photocathode, an exit window and an exit phosphor screen.
2. Description of the Related Art
An X-ray image intensifier tube of this kind is known from U.S. Pat. No. 4,831,249.
In known X-ray image intensifier tubes, having an entrance screen which includes a luminescent layer and a photocathode, improvement of a selected property, for example the efficiency or image information transfer, usually is at the expense of other properties or of at least one other property. For example, a separating layer between the luminescent layer and the photocathode as described in U.S. Pat. No. 4,831,249, can improve the image transfer from the entrance screen, but will also lead to a loss of efficiency as mentioned therein.
It is to be noted that a luminescent screen comprising two different phosphors is known per se from U.S. Pat. No. 4,712,011, but therein the phosphors are mixed or provided in different volume parts, transversely of the layer thickness, so that a loss of resolution is liable to occur. The second layer notably consists of CsI and the first layer consists of a phosphor having a comparatively high absorption for the 35 keV K radiation of the CsI, such as Ca WO.sub.4, bismuth germanate or combinations thereof.